TOKYO — According to its official statements, North Korea is ready to go to the brink. But how serious are Pyongyang’s threats?

This week, new U.N. sanctions punishing the North’s successful December rocket launch have elicited a furious response from Pyongyang: strong hints that a third nuclear test is coming, along with bigger and better long-range missiles; “all-out action” against its “sworn enemy,” the United States; and on Friday, a threat of “strong physical countermeasures” against South Korea if Seoul participates in the sanctions.

“Sanctions mean war,” said a statement carried by North Korea’s official Korean Central News Agency.

A U.S. research institute said Friday that recent satellite photos of the site where nuclear tests were conducted in 2006 and 2009 show North Korea could be almost ready to carry out its threat.

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In the face of international condemnation, North Korea can usually be counted on for such flights of rhetorical pique. In recent years it threatened to turn South Korea into a “sea of fire,” and to wage a “sacred war” against its enemies.

If the past is any indication, its threats of war are overblown. But the chances it will conduct another nuclear test are high. And it is gaining ground in its missile program, experts say, though still a long way from seriously threatening the U.S. mainland.

“It’s not the first time they’ve made a similar threat of war,” said Ryoo Kihl-jae, a professor at the University of North Korean Studies in Seoul. “What’s more serious than the probability of an attack on South Korea is that of a nuclear test. I see very slim chances of North Korea following through with its threat of war.”

Although North Korea’s leadership is undeniably concerned that it might be attacked or bullied by outside powers, the tough talk is mainly an attempt to bolster its bargaining position in diplomatic negotiations.

The impoverished North is in need of international aid and is eager to sign a treaty bringing a formal end to the Korean War, which ended nearly 60 years ago in a truce. It uses its weapons program as a wedge in the ever-repeating diplomatic dance with the U.S.-led international community, and there is no reason to believe this time is different.

“I see this as their way of testing the water,” said Narushige Michishita, a North Korea expert at Tokyo’s Graduate Institute of Policy Studies. “North Korea will probably never be able to defeat the United States in a war. But they are getting stronger.”

In 2006 and 2009, North Korea carried out underground nuclear tests just after receiving U.N. sanctions for launching long-range rockets. The latest barrage of rhetoric comes after the U.N. Security Council voted unanimously on Tuesday to condemn the successful Dec. 12 rocket launch and further expand sanctions against Kim Jong Un’s regime. Pyongyang replied with its threat of more launches and possibly another nuclear test.

“Settling accounts with the U.S. needs to be done with force, not with words,” said Thursday’s statement from the National Defense Commission, which promised “a new phase of the anti-U.S. struggle that has lasted century after century.”

North Korea has long insisted that its rocket launches were peaceful attempts to put a satellite in orbit, while the U.S. and United Nations consider them illegal tests of missile technology. This week, however, Pyongyang, made it clear that one goal of its rocket program is to attack the United States.

But its ability to do so is limited, say experts who believe North Korea still has technological kinks to work out in its nuclear devices. It is thought to be unable to make a nuclear weapon small enough to be mounted on a missile, so it needs to test that technology as well.

Another big issue is money.

In his first speech to his people, the young leader, Kim, who is still believed to be in his 20s, said North Korea will continue its “military first” policy. But for a nation that chronically struggles to feed its own people, resources are limited. And because of trade restrictions, acquiring parts for its weapons from abroad is increasingly difficult.

Despite December’s successful launch, North Korea’s ability to get missiles off the launch pad is less than reliable. In April, a similar rocket splintered into pieces over the Yellow Sea. Days later, North Korea showed off what appeared to be an intercontinental ballistic missile, but many experts who reviewed footage of the rockets said they were clearly fakes.

The North does, however, appear to be making some progress.

Japan’s Defense Ministry, in an assessment of the December launch presented to the prime minister on Friday, said the North’s best designs probably give its missiles a range of more than 10,000 kilometers (6,200 miles), according to Japan’s Kyodo news service. That would be enough to reach the West Coast of the United States. A South Korean defense official said Friday that Seoul agrees with that assessment.

The Japanese report warned that Pyongyang’s missile technology has “entered a new stage” that is of serious concern to the international community. Japan is particularly wary of North Korea’s capabilities because all of its islands are well within striking distance. Japan also hosts about 50,000 U.S. troops, whose bases would be a tempting target if Pyongyang were to try to make good on its threats.

“There has been a tendency to underestimate what North Korea can do in the space and missile field, and possibly with technology in general,” U.S. nuclear expert Jeffrey Lewis wrote recently on his Arms Control Wonk blog. He noted that debris recovered from the wreckage of the December rocket’s first stage indicates that most of it was made in North Korea.

North Korea claims the right to build nuclear weapons as a defense against the United States, which stations more than 28,000 troops in South Korea.

It is believed to have enough weapons-grade plutonium for about four to eight bombs, according to nuclear scientist Siegfried Hecker, who visited North Korea’s nuclear complex in 2010. And in 2009, Pyongyang also declared that it would begin enriching uranium, giving it a second way to make atomic weapons.

U.S. Defense Secretary Leon Panetta said Thursday that he has seen no outward sign that North Korea will follow through soon on its plan to conduct a test, but added that doesn’t mean preparations aren’t under way.

A U.S. research institute said Friday that recent satellite photos of the Punggye-ri site where nuclear tests were conducted in 2006 and 2009 reveal that over the past month roads have been kept clear of snow and that North Koreans may have been sealing the tunnel into a mountainside where a nuclear device would be detonated.

The analysis was provided to The Associated Press by 38 North, the website of U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies. The latest image was taken Wednesday.

38 North concludes that the Punggye-ri site “appears to continue to be at a state of readiness that would allow the North to move forward with a test in a few weeks or less once the leadership in Pyongyang gives the order.”

U.S. officials confirmed Friday that the U.S. has seen some trucks moving around the site. One official said the U.S. is not ruling out that the test could happen in the near future.

But the officials cautioned that, as in previous tests, because it would be done underground, the U.S. may not know much before it actually happens. Officials spoke on condition of anonymity because they were not authorized to discuss intelligence matters publicly.